1. Field of the Invention
The present invention relates to a phase error detection apparatus and a phase error detection method for detecting the phase error of PLL, as well as to a reproduction apparatus for reproducing data from a recording medium on which bit information is recorded.
2. Description of the Related Art
References should be made to Japanese Patent Laid-Open No. Hei 8-69672 and Japanese Patent Laid-Open No. 2003-6864.
As optical recording media from which recorded signals are reproduced by irradiation of light, so-called high recording density optical disks such as BD (Blu-ray Disk: registered trademark) are currently used extensively.
In order to reproduce recorded information from this type of optical disk, there may be carried out PRML (partial response maximum likelihood) decoding.
To execute PRML decoding involves selecting what is known as a PR class corresponding to the characteristics (mainly recording density) of the recording and reproduction system in use. Well-known PR classes include PR(1, 2, 1) and PR(1, 2, 2, 1).
Meanwhile, maintaining a stable PLL (phase locked loop) setup is essential for allowing PRML decoding to exert its full potential.
In ordinary optical disk systems, phase error information about the PLL is often acquired from values in the vicinity of a zero cross point of the reproduced signal. Two typical techniques of obtaining the phase error information, to be explained below, have been known.
FIGS. 11A and 11B are schematic views explanatory of the ordinary techniques of detecting phase errors from values in the vicinity of a zero cross point of the reproduced signal. FIG. 11A is explanatory of the detection technique in effect when the ideal value of the reproduced signal is not zero. FIG. 11B is explanatory of the detection technique for use when the ideal value of the reproduced signal is zero.
If PR(1, 2, 1) is adopted as the PR class, then the ideal value of the reproduced signal following PR equalization is not zero. In other words, the timing for ideal sampling is at a point different from the zero cross point. The technique explained in reference to FIG. 11A is thus the phase error detection technique to be adopted where the ideal value of the reproduced signal is other than zero with PR(1, 2, 1) in effect.
If PR(1, 2, 2, 1) is adopted, then the ideal value of the reproduced signal following PR equalization is zero. That is, the zero cross point coincides ideally with the sampling point. The technique explained in reference to FIG. 11B is thus the phase error detection technique to be adopted where the ideal value of the reproduced signal is zero with PR(1, 2, 2, 1) in effect.
When the ideal value of the reproduced signal is other than zero as shown in FIG. 11A, a phase error Δτ is obtained using the following expression:Δτ=sign*(An−1+An)  [Expression 1]where, An−1 denotes the sampling value before a zero cross of the reproduced signal, and An represents the sampling value after the zero cross of the reproduced signal.
In the above expression, “sign” is either “+” or “−” depending on the zero cross direction (from positive to negative or vice versa).
If PR(1, 2, 1) is adopted, the absolute value of the sampling value An−1 before the zero cross is ideally the same as the absolute value of the sampling value An after the zero cross. Thus adding up the values An−1 and An as in the above expression provides a value indicating both the amount of the error from an ideal phase and the polarity of that error (i.e., phase advanced or delayed).
FIG. 11A shows a state in which zero is crossed in the positive-to-negative direction. If zero is crossed conversely in the negative-to-positive direction, then the relation between the polarity of the value calculated by “An−1+An” and the advance/delay of the phase is inverse to what is shown in FIG. 11A. The “sign” in the above expression is used to correct the incorrect sign (i.e., polarity) of the value “An−1+An” stemming from the difference of the zero-cross direction.
Where the ideal value of the reproduced signal is zero as shown in FIG. 11B, the phase error Δτ is detected based on the concept explained hereunder.
FIG. 11B shows three states: one in which the phase error in the waveform of the reproduced signal at a zero cross is zero (waveform in the middle), a state in which the phase is advanced (waveform on the left), and a state in which the phase is delayed (waveform on the right).
It is reaffirmed here that the phase error of the PLL occurs as a difference from the ideal point of the sampling timing for the reproduced signal. If the phase error were to be indicated faithfully in terms of the difference of the sampling timing on the same reproduced signal, the phenomenon would be too complicated to illustrate. For purpose of simplification and illustration, the sampling timing is thus shown in the three phase states: ideal, advanced, and delayed, depicted individually in waveforms.
In the figures, the sampling timing (i.e., sampling point) on the time axis, illustratively while the phase is being advanced, is indicated as solid points at which the waveform of the reproduced signal shifted to the right as viewed on the plan view of the figures coincides with the waveform of the reproduced signal in the ideal state with the phase advanced. In like manner, the sampling timing in effect while the phase is being delayed is indicated as solid points at which the waveform of the reproduced signal shifted to the left as viewed on the plan view of the figures coincides with the waveform of the reproduced signal in the ideal state with the phase delayed.
At this point, the zero cross point of the reproduced signal in the ideal state is assumed to be a reference sampling point An. In reference to the sampling point An, the sampling point before the zero cross of the reproduced signal in the ideal state is assumed to be An−1, and the sampling point after the zero cross is assumed to be An+1.
FIG. 11B shows the sampling points corresponding to the ideal, advanced, and delayed states individually. In this setup, where the phase is advanced, the sampling point before the zero cross is indicated in FIG. 11B as An−1 and the sampling point after the zero cross as An. Where the phase is delayed, the sampling point before the zero cross is An and the sampling point after the zero cross is An+1.
In the case of FIG. 11B, the value of the sampling point An in the ideal state where there is no phase error is zero. As will be understood from this, the value of the sampling point An indicates the phase error with PR(1, 2, 2, 1) in effect where the ideal value of the reproduced signal is zero.
That is, where the ideal value of the reproduced signal is zero and where the phase is advanced, the phase error Δτ is given by the following expression:Δτ=sign*min(An,An−1)  [Expression 2]where, An−1 denotes the value of the sampling point before the zero cross and An represents the value of the sampling point after the zero cross.
Where the phase is delayed, the phase error Δτ is given by the following expression:Δτ=sign*min(An+1,An)  [Expression 3]where, An denotes the value of the sampling point before the zero cross and An+1 represents the value of the sampling point after the zero cross.
In the above expressions 2 and 3, the term “min(x, y)” constitutes an operator for selecting either “x” or “y,” which is the smaller of the two in absolute value.
An actual circuit determines whether the reproduced signal has reached a zero cross by comparing the current value of the reproduced signal with the immediately preceding value thereof to see if the polarity has changed from one value to the other. In view of this technique of detecting the zero cross point, the above-described expressions 2 and 3 may be arranged into the following single expression 4:Δτ=sign*min(Ak,Ak−1)  [Expression 4]where, Ak denotes the value of the sampling point at which the zero cross is detected and Ak−1 represents the value of the immediately preceding sampling point.